C.M.
My almost 4 year old is doing this too, and I had trouble with my 6 year old when she was about this age. What I'm struggling with now is a)is he really lying (I have 4 kids, I can't always know who drew on the wall or didn't flush) and b)does he truly understand what a lie is. My friend suggested I use easy black and white things (literally, she suggested colors) to make sure he knew the difference between the truth and falsehood.
If I know for a fact that he is lying, I use Tabasco sauce on his tongue. He hates it. I used this for my daughter as well and truly, I only used it twice before she got the message that lying was unacceptable. I also use it for bad words, disrespectful talk and biting - pretty much any time they use their mouth in a way that God didn't intend. I worry sometimes I will ruin their taste for spicy food when they associate that bottle with punishment...I love the vinegar idea - I may switch.